Technical Field
The invention relates to methods for transmitting channel quality reports and/or sounding reference symbols from a mobile station to a base station. The invention is also providing the mobile station and the base station for performing the methods described herein.
Description of the Related Art
Long Term Evolution (LTE)
Third-generation mobile systems (3G) based on WCDMA radio-access technology are being deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio access technology that is highly competitive.
In order to be prepared for further increasing user demands and to be competitive against new radio access technologies, 3GPP introduced a new mobile communication system which is called Long Term Evolution (LTE). LTE is designed to meet the carrier needs for high speed data and media transport as well as high capacity voice support for the next decade. The ability to provide high bit rates is a key measure for LTE.
The work item (WI) specification on Long-Term Evolution (LTE) called Evolved UMTS Terrestrial Radio Access (UTRA) and UMTS Terrestrial Radio Access Network (UTRAN) is finalized as Release 8 (LTE Rel. 8). The LTE system represents efficient packet-based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. In LTE, scalable multiple transmission bandwidths are specified such as 1.4, 3.0, 5.0, 10.0, 15.0, and 20.0 MHz, in order to achieve flexible system deployment using a given spectrum. In the downlink, Orthogonal Frequency Division Multiplexing (OFDM) based radio access was adopted because of its inherent immunity to multipath interference (MPI) due to a low symbol rate, the use of a cyclic prefix (CP) and its affinity to different transmission bandwidth arrangements. Single-carrier frequency division multiple access (SC-FDMA) based radio access was adopted in the uplink, since provisioning of wide area coverage was prioritized over improvement in the peak data rate considering the restricted transmit power of the user equipment (UE). Many key packet radio access techniques are employed including multiple-input multiple-output (MIMO) channel transmission techniques and a highly efficient control signaling structure is achieved in LTE Rel. 8/9.
LTE Architecture
The overall architecture is shown in FIG. 1 and a more detailed representation of the E-UTRAN architecture is given in FIG. 2. The E-UTRAN consists of an eNodeB, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the user equipment (UE). The eNodeB (eNB) hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC) and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. It performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated uplink Quality of Service (QoS), cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of downlink/uplink user plane packet headers. The eNodeBs are interconnected with each other by means of the X2 interface.
The eNodeBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (SGW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNodeBs. The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GW). For idle state user equipments, the SGW terminates the downlink data path and triggers paging when downlink data arrives for the user equipment. It manages and stores user equipment contexts, e.g., parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME is the key control-node for the LTE access-network. It is responsible for idle mode user equipment tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a user equipment at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS). The Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to user equipments. It checks the authorization of the user equipment to camp on the service provider's Public Land Mobile Network (PLMN) and enforces user equipment roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming user equipments.
Component Carrier Structure in LTE (Release 8)
The downlink component carrier of a 3GPP LTE (Release 8) is subdivided in the time-frequency domain in so-called subframes. In 3GPP LTE (Release 8) each subframe is divided into two downlink slots as shown in FIG. 3, wherein the first downlink slot comprises the control channel region (PDCCH region) within the first OFDM symbols. Each subframe consists of a give number of OFDM symbols in the time domain (12 or 14 OFDM symbols in 3GPP LTE (Release 8)), wherein each OFDM symbol spans over the entire bandwidth of the component carrier. The OFDM symbols thus each consists of a number of modulation symbols transmitted on respective NRBDL×NscRB subcarriers as also shown in FIG. 4.
Assuming a multi-carrier communication system, e.g., employing OFDM, as for example used in 3GPP Long Term Evolution (LTE), the smallest unit of resources that can be assigned by the scheduler is one “resource block”. A physical resource block (PRB) is defined as NsymbDL consecutive OFDM symbols in the time domain (e.g., 7 OFDM symbols) and NscRB consecutive subcarriers in the frequency domain as exemplified in FIG. 4 (e.g., 12 subcarriers for a component carrier). In 3GPP LTE (Release 8), a physical resource block thus consists of NsymbDL×NscRB resource elements, corresponding to one slot in the time domain and 180 kHz in the frequency domain (for further details on the downlink resource grid, see for example 3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)”, section 6.2, available at http://www.3gpp.org and incorporated herein by reference).
One subframe consists of two slots, so that there are 14 OFDM symbols in a subframe when a so-called “normal” CP (cyclic prefix) is used, and 12 OFDM symbols in a subframe when a so-called “extended” CP is used. For sake of terminology, in the following the time-frequency resources equivalent to the same NscRB consecutive subcarriers spanning a full subframe is called a “resource block pair”, or equivalent “RB pair” or “PRB pair”.
The term “component carrier” refers to a combination of several resource blocks in the frequency domain. In future releases of LTE, the term “component carrier” is no longer used; instead, the terminology is changed to “cell”, which refers to a combination of downlink and optionally uplink resources. The linking between the carrier frequency of the downlink resources and the carrier frequency of the uplink resources is indicated in the system information transmitted on the downlink resources.
Similar assumptions for the component carrier structure apply to later releases too.
Carrier Aggregation in LTE-A for Support of Wider Bandwidth
The frequency spectrum for IMT-Advanced was decided at the World Radio communication Conference 2007 (WRC-07). Although the overall frequency spectrum for IMT-Advanced was decided, the actual available frequency bandwidth is different according to each region or country. Following the decision on the available frequency spectrum outline, however, standardization of a radio interface started in the 3rd Generation Partnership Project (3GPP). At the 3GPP TSG RAN #39 meeting, the Study Item description on “Further Advancements for E-UTRA (LTE-Advanced)” was approved. The study item covers technology components to be considered for the evolution of E-UTRA, e.g., to fulfill the requirements on IMT-Advanced.
The bandwidth that the LTE-Advanced system is able to support is 100 MHz, while an LTE system can only support 20 MHz. Nowadays, the lack of radio spectrum has become a bottleneck of the development of wireless networks, and as a result it is difficult to find a spectrum band which is wide enough for the LTE-Advanced system. Consequently, it is urgent to find a way to gain a wider radio spectrum band, wherein a possible answer is the carrier aggregation functionality.
In carrier aggregation, two or more component carriers (component carriers) are aggregated in order to support wider transmission bandwidths up to 100 MHz. Several cells in the LTE system are aggregated into one wider channel in the LTE-Advanced system which is wide enough for 100 MHz even though these cells in LTE are in different frequency bands.
All component carriers can be configured to be LTE Rel. 8/9 compatible, at least when the aggregated numbers of component carriers in the uplink and the downlink are the same. Not all component carriers aggregated by a user equipment may necessarily be Rel. 8/9 compatible. Existing mechanism (e.g., barring) may be used to avoid Rel-8/9 user equipments to camp on a component carrier.
A user equipment may simultaneously receive or transmit one or multiple component carriers (corresponding to multiple serving cells) depending on its capabilities. A LTE-A Rel. 10 user equipment with reception and/or transmission capabilities for carrier aggregation can simultaneously receive and/or transmit on multiple serving cells, whereas an LTE Rel. 8/9 user equipment can receive and transmit on a single serving cell only, provided that the structure of the component carrier follows the Rel. 8/9 specifications.
Carrier aggregation is supported for both contiguous and non-contiguous component carriers with each component carrier limited to a maximum of 110 Resource Blocks in the frequency domain using the 3GPP LTE (Release 8/9) numerology.
It is possible to configure a 3GPP LTE-A (Release 10) compatible user equipment to aggregate a different number of component carriers originating from the same eNodeB (base station) and of possibly different bandwidths in the uplink and the downlink. The number of downlink component carriers that can be configured depends on the downlink aggregation capability of the UE. Conversely, the number of uplink component carriers that can be configured depends on the uplink aggregation capability of the UE. It may not be possible to configure a mobile terminal with more uplink component carriers than downlink component carriers.
In a typical TDD deployment, the number of component carriers and the bandwidth of each component carrier in uplink and downlink is the same. Component carriers originating from the same eNodeB need not to provide the same coverage.
The spacing between center frequencies of contiguously aggregated component carriers shall be a multiple of 300 kHz. This is in order to be compatible with the 100 kHz frequency raster of 3GPP LTE (Release 8/9) and at the same time preserve orthogonality of the subcarriers with 15 kHz spacing. Depending on the aggregation scenario, the n×300 kHz spacing can be facilitated by insertion of a low number of unused subcarriers between contiguous component carriers.
The nature of the aggregation of multiple carriers is only exposed up to the MAC layer. For both uplink and downlink there is one HARQ entity required in MAC for each aggregated component carrier. There is (in the absence of SU-MIMO for uplink) at most one transport block per component carrier. A transport block and its potential HARQ retransmissions need to be mapped on the same component carrier.
The Layer 2 structure with activated carrier aggregation is shown in FIG. 5 and FIG. 6 for the downlink and uplink respectively.
When carrier aggregation is configured, the mobile terminal only has one RRC connection with the network. At RRC connection establishment/re-establishment, one cell provides the security input (one ECGI, one PCI and one ARFCN) and the non-access stratum mobility information (e.g., TAI) similarly as in LTE Rel. 8/9. After RRC connection establishment/re-establishment, the component carrier corresponding to that cell is referred to as the downlink Primary Cell (PCell). There is always one and only one downlink PCell (DL PCell) and one uplink PCell (UL PCell) configured per user equipment in connected state. Within the configured set of component carriers, other cells are referred to as Secondary Cells (SCells); with carriers of the SCell being the Downlink Secondary Component Carrier (DL SCC) and Uplink Secondary Component Carrier (UL SCC). The characteristics of the downlink and uplink PCell are:
For each SCell the usage of uplink resources by the UE, in addition to the downlink ones is configurable; the number of DL SCCs configured is therefore always larger or equal to the number of UL SCCs, and no SCell can be configured for usage of uplink resources only
The uplink PCell is used for transmission of Layer 1 uplink control information
The downlink PCell cannot be de-activated, unlike SCells
From UE perspective, each uplink resource only belongs to one serving cell
The number of serving cells that can be configured depends on the aggregation capability of the UE
Re-establishment is triggered when the downlink PCell experiences Rayleigh fading (RLF), not when downlink SCells experience RLF
The downlink PCell cell can change with handover (i.e., with security key change and RACH procedure)
Non-access stratum information is taken from the downlink PCell
PCell can only be changed with handover procedure (i.e., with security key change and RACH procedure)
PCell is used for transmission of PUCCH
The configuration and reconfiguration of component carriers can be performed by RRC. Activation and deactivation is done via MAC control elements. At intra-LTE handover, RRC can also add, remove, or reconfigure SCells for usage in the target cell. When adding a new SCell, dedicated RRC signaling is used for sending the system information of the SCell, the information being necessary for transmission/reception (similarly as in Rel-8/9 for handover).
When a user equipment is configured with carrier aggregation there is one pair of uplink and downlink component carriers that is always active. The downlink component carrier of that pair might be also referred to as ‘DL anchor carrier’. Same applies also for the uplink.
When carrier aggregation is configured, a user equipment may be scheduled over multiple component carriers simultaneously but at most one random access procedure shall be ongoing at any time. Cross-carrier scheduling allows the PDCCH of a component carrier to schedule resources on another component carrier. For this purpose a component carrier identification field is introduced in the respective DCI formats, called CIF.
A linking between uplink and downlink component carriers allows identifying the uplink component carrier for which the grant applies when there is no-cross-carrier scheduling. The linkage of downlink component carriers to uplink component carrier does not necessarily need to be one to one. In other words, more than one downlink component carrier can link to the same uplink component carrier. At the same time, a downlink component carrier can only link to one uplink component carrier.
LTE RRC States
LTE is based on only two main states: “RRC_IDLE” and “RRC_CONNECTED”.
In RRC_IDLE the radio is not active, but an ID is assigned and tracked by the network. More specifically, a mobile terminal in RRC_IDLE performs cell selection and reselection—in other words, it decides on which cell to camp. The cell (re)selection process takes into account the priority of each applicable frequency of each applicable Radio Access Technology (RAT), the radio link quality and the cell status (i.e., whether a cell is barred or reserved). An RRC_IDLE mobile terminal monitors a paging channel to detect incoming calls, and also acquires system information. The system information mainly consists of parameters by which the network (E-UTRAN) can control the cell (re)selection process. RRC specifies the control signaling applicable for a mobile terminal in RRC_IDLE, namely paging and system information. The mobile terminal behavior in RRC_IDLE is specified in TS 36.304, incorporated herein by reference.
In RRC_CONNECTED the mobile terminal has an established RRC connection with contexts in the eNodeB. The E-UTRAN allocates radio resources to the mobile terminal to facilitate the transfer of (unicast) data via shared data channels. To support this operation, the mobile terminal monitors an associated control channel which is used to indicate the dynamic allocation of the shared transmission resources in time and frequency. The mobile terminal provides the network with reports of its buffer status and of the downlink channel quality, as well as neighboring cell measurement information to enable E-UTRAN to select the most appropriate cell for the mobile terminal. These measurement reports include cells using other frequencies or RATs. The UE also receives system information, consisting mainly of information required to use the transmission channels. To extend its battery lifetime, a UE in RRC_CONNECTED may be configured with a Discontinuous Reception (DRX) cycle. RRC is the protocol by which the E-UTRAN controls the UE behavior in RRC_CONNECTED.
FIG. 7 shows a state diagram with an overview of the relevant functions performed by the mobile terminal in IDLE and CONNECTED state.
Logical and Transport Channels
The MAC layer provides a data transfer service for the RLC layer through logical channels. Logical channels are either Control Logical Channels which carry control data such as RRC signaling, or Traffic Logical Channels which carry user plane data. Broadcast Control Channel (BCCH), Paging Control channel (PCCH), Common Control Channel (CCCH), Multicast Control Channel (MCCH) and Dedicated Control Channel (DCCH) are Control Logical Channels. Dedicated Traffic channel (DTCH) and Multicast Traffic Channel (MTCH) are Traffic Logical Channels.
Data from the MAC layer is exchanged with the physical layer through Transport Channels. Data is multiplexed into transport channels depending on how it is transmitted over the air. Transport channels are classified as downlink or uplink as follows. Broadcast Channel (BCH), Downlink Shared Channel (DL-SCH), Paging Channel (PCH) and Multicast Channel (MCH) are downlink transport channels, whereas the Uplink Shared Channel (UL-SCH) and the Random Access Channel (RACH) are uplink transport channels.
A multiplexing is then performed between logical channels and transport channels in the downlink and uplink respectively.
Layer 1/Layer 2 (L1/L2) Control Signaling
In order to inform the scheduled users about their allocation status, transport format and other data-related information (e.g., HARQ information, transmit power control (TPC) commands), L1/L2 control signaling is transmitted on the downlink along with the data. L1/L2 control signaling is multiplexed with the downlink data in a subframe, assuming that the user allocation can change from subframe to subframe. It should be noted that user allocation might also be performed on a TTI (Transmission Time Interval) basis, where the TTI length can be a multiple of the subframes. The TTI length may be fixed in a service area for all users, may be different for different users, or may even by dynamic for each user. Generally, the L1/L2 control signaling needs only be transmitted once per TTI. Without loss of generality, the following assumes that a TTI is equivalent to one subframe.
The L1/L2 control signaling is transmitted on the Physical Downlink Control Channel (PDCCH). A PDCCH carries a message as a Downlink Control Information (DCI), which includes resource assignments and other control information for a mobile terminal or groups of UEs. In general, several PDCCHs can be transmitted in one subframe.
It should be noted that in 3GPP LTE, assignments for uplink data transmissions, also referred to as uplink scheduling grants or uplink resource assignments, are also transmitted on the PDCCH.
With respect to scheduling grants, the information sent on the L1/L2 control signaling may be separated into the following two categories, Shared Control Information (SCI) carrying Cat 1 information and Downlink Control Information (DCI) carrying Cat 2/3 information.
Shared Control Information (SCI) Carrying Cat 1 Information
The shared control information part of the L1/L2 control signaling contains information related to the resource allocation (indication). The shared control information typically contains the following information:
A user identity indicating the user(s) that is/are allocated the resources.
RB allocation information for indicating the resources (Resource Blocks (RBs)) on which a user(s) is/are allocated. The number of allocated resource blocks can be dynamic.
The duration of assignment (optional), if an assignment over multiple sub-frames (or TTIs) is possible.
Depending on the setup of other channels and the setup of the Downlink Control Information (DCI)—see below—the shared control information may additionally contain information such as ACK/NACK for uplink transmission, uplink scheduling information, information on the DCI (resource, MCS, etc.).
Downlink Control Information (DCI) Carrying Cat 2/3 Information
The downlink control information part of the L1/L2 control signaling contains information related to the transmission format (Cat 2 information) of the data transmitted to a scheduled user indicated by the Cat 1 information. Moreover, in case of using (Hybrid) ARQ as a retransmission protocol, the Cat 2 information carries HARQ (Cat 3) information. The downlink control information needs only to be decoded by the user scheduled according to Cat 1. The downlink control information typically contains information on:
Cat 2 information: Modulation scheme, transport-block (payload) size or coding rate, MIMO (Multiple Input Multiple Output)-related information, etc. Either the transport-block (or payload size) or the code rate can be signaled. In any case these parameters can be calculated from each other by using the modulation scheme information and the resource information (number of allocated resource blocks)
Cat 3 information: HARQ related information, e.g., hybrid ARQ process number, redundancy version, retransmission sequence number
Downlink control information occurs in several formats that differ in overall size and also in the information contained in its fields. The different DCI formats that are currently defined for LTE are described in detail in 3GPP TS 36.212, “Multiplexing and channel coding”, section 5.3.3.1 (available at http://www.3gpp.org and incorporated herein by reference).
Uplink Control Information (UCI)
In general, uplink control signaling in mobile communication systems can be divided into two categories:
Data-associated control signaling, is control signaling which is always transmitted together with uplink data and is used in the processing of that data. Examples include transport format indications, “New data” Indicator (NDIs) and MIMO parameters.
Control signaling not associated with data is transmitted independently of any uplink data packet. Examples include HARQ Acknowledgements (ACK/NACK) for downlink data packets, Channel Quality Indicators (CQIs) to support link adaptation, and MIMO feedback such as Rank Indicators (RIs) and Precoding Matrix Indicators (PMI) for downlink transmissions. Scheduling Requests (SRs) for uplink transmissions also fall into this category.
Uplink data-associated control signaling is not necessary in LTE, as the relevant information is already known to the eNodeB. Therefore, only data-non-associated control signaling exists in the LTE uplink.
Consequently, the UCI can consist of:
Scheduling Requests (SRs)
HARQ ACK/NACK in response to downlink data packets on the PDSCH (Physical Downlink Shared CHannel). One ACK/NACK bit is transmitted in the case of single-codeword downlink transmission while two ACK/NACK bits are used in the case of two-codeword downlink transmission.
Channel State Information (CSI) which includes CQIs as well as the MIMO-related feedback consisting of RIs and PMI. 20 bits per subframe are used for the CSI.
The amount of UCI a UE can transmit in a subframe depends on the number of SC-FDMA symbols available for transmission of control signaling data. The PUCCH supports eight different formats, depending on the amount of information to be signaled. The following UCI formats on PUCCH are supported, according to the following overview.
PUCCH FormatUplink Control Information (UCI)Format 1Scheduling Request (SR) (unmodulated waveform)Format 1a1-bit HARQ ACK/NACK with/without SRFormat 1b2-bit HARQ ACK/NACK with/without SRFormat 2CSI (20 coded bits)Format 2CSI and 1- or 2-bit HARQ ACK/NACK for extendedCP onlyFormat 2aCSI and 1-bit HARQ ACK/NACK (20 + 1 coded bits)Format 2bCSI and 2-bit HARQ ACK/NACK (20 + 2 coded bits)Format 3Multiple ACK/NACKs for carrier aggregation: up to20 ACK/NACK bits plus optional SR, in 48 coded bits
Using the different defined PUCCH formats (according to 5.4.1 and 5.4.2 of TS 36.211), the following combinations of UCI on PUCCH are supported (see Section 10.1.1 of TS 36.213):
Format 1a for 1-bit HARQ-ACK or in case of FDD for 1-bit HARQ-ACK with positive SR
Format 1b for 2-bit HARQ-ACK or for 2-bit HARQ-ACK with positive SR
Format 1b for up to 4-bit HARQ-ACK with channel selection when the UE is configured with more than one serving cell or, in the case of TDD, when the UE is configured with a single serving cell
Format 1 for positive SR
Format 2 for a CSI report when not multiplexed with HARQ-ACK
Format 2a for a CSI report multiplexed with 1-bit HARQ-ACK for normal cyclic prefix
Format 2b for a CSI report multiplexed with 2-bit HARQ-ACK for normal cyclic prefix
Format 2 for a CSI report multiplexed with HARQ-ACK for extended cyclic prefix
Format 3 for up to 10-bit HARQ-ACK for FDD and for up to 20-bit HARQ-ACK for TDD
Format 3 for up to 11-bit corresponding to 10-bit HARQ-ACK and 1-bit positive/negative SR for FDD and for up to 21-bit corresponding to 20-bit HARQ-ACK and 1-bit positive/negative SR for TDD.
Format 3 for multi-cell HARQ-ACK, 1-bit positive/negative SR and a CSI report for one serving cell.
Downlink & Uplink Data Transmission
Regarding downlink data transmission, L1/L2 control signaling is transmitted on a separate physical channel (PDCCH), along with the downlink packet data transmission. This L1/L2 control signaling typically contains information on:
The physical resource(s) on which the data is transmitted (e.g., subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA). This information allows the mobile terminal (receiver) to identify the resources on which the data is transmitted.
When user equipment is configured to have a Carrier Indication Field (CIF) in the L1/L2 control signaling, this information identifies the component carrier for which the specific control signaling information is intended. This enables assignments to be sent on one component carrier which are intended for another component carrier (“cross-carrier scheduling”). This other, cross-scheduled component carrier could be for example a PDCCH-less component carrier, i.e., the cross-scheduled component carrier does not carry any L1/L2 control signaling.
The Transport Format, which is used for the transmission. This can be the transport block size of the data (payload size, information bits size), the MCS (Modulation and Coding Scheme) level, the Spectral Efficiency, the code rate, etc. This information (usually together with the resource allocation (e.g., the number of resource blocks assigned to the user equipment)) allows the user equipment (receiver) to identify the information bit size, the modulation scheme and the code rate in order to start the demodulation, the de-rate-matching and the decoding process. The modulation scheme may be signaled explicitly.
Hybrid ARQ (HARQ) information:
HARQ process number: Allows the user equipment to identify the hybrid ARQ process on which the data is mapped.
Sequence number or new data indicator (NDI): Allows the user equipment to identify if the transmission is a new packet or a retransmitted packet. If soft combining is implemented in the HARQ protocol, the sequence number or new data indicator together with the HARQ process number enables soft-combining of the transmissions for a PDU prior to decoding.
Redundancy and/or constellation version: Tells the user equipment, which hybrid ARQ redundancy version is used (required for de-rate-matching) and/or which modulation constellation version is used (required for demodulation).
UE Identity (UE ID): Tells for which user equipment the L1/L2 control signaling is intended for. In typical implementations this information is used to mask the CRC of the L1/L2 control signaling in order to prevent other user equipments to read this information.
To enable an uplink packet data transmission, L1/L2 control signaling is transmitted on the downlink (PDCCH) to tell the user equipment about the transmission details. This L1/L2 control signaling typically contains information on:
The physical resource(s) on which the user equipment should transmit the data (e.g., subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA).
When user equipment is configured to have a Carrier Indication Field (CIF) in the L1/L2 control signaling, this information identifies the component carrier for which the specific control signaling information is intended. This enables assignments to be sent on one component carrier which are intended for another component carrier. This other, cross-scheduled component carrier may be for example a PDCCH-less component carrier, i.e., the cross-scheduled component carrier does not carry any L1/L2 control signaling.
L1/L2 control signaling for uplink grants is sent on the DL component carrier that is linked with the uplink component carrier or on one of the several DL component carriers, if several DL component carriers link to the same UL component carrier.
The Transport Format, the user equipment should use for the transmission. This can be the transport block size of the data (payload size, information bits size), the MCS (Modulation and Coding Scheme) level, the Spectral Efficiency, the code rate, etc. This information (usually together with the resource allocation (e.g., the number of resource blocks assigned to the user equipment)) allows the user equipment (transmitter) to pick the information bit size, the modulation scheme and the code rate in order to start the modulation, the rate-matching and the encoding process. In some cases the modulation scheme maybe signaled explicitly.
Hybrid ARQ information:
HARQ Process number: Tells the user equipment from which hybrid ARQ process it should pick the data.
Sequence number or new data indicator: Tells the user equipment to transmit a new packet or to retransmit a packet. If soft combining is implemented in the HARQ protocol, the sequence number or new data indicator together with the HARQ process number enables soft-combining of the transmissions for a protocol data unit (PDU) prior to decoding.
Redundancy and/or constellation version: Tells the user equipment, which hybrid ARQ redundancy version to use (required for rate-matching) and/or which modulation constellation version to use (required for modulation).
UE Identity (UE ID): Tells which user equipment should transmit data. In typical implementations this information is used to mask the CRC of the L1/L2 control signaling in order to prevent other user equipments to read this information.
There are several different possibilities how to exactly transmit the information pieces mentioned above in uplink and downlink data transmission.
Moreover, in uplink and downlink, the L1/L2 control information may also contain additional information or may omit some of the information. For example:
HARQ process number may not be needed, i.e., is not signaled, in case of a synchronous HARQ protocol.
A redundancy and/or constellation version may not be needed, and thus not signaled, if Chase Combining is used (always the same redundancy and/or constellation version) or if the sequence of redundancy and/or constellation versions is pre-defined.
Power control information may be additionally included in the control signaling.
MIMO related control information, such as, e.g., pre-coding, may be additionally included in the control signaling.
In case of multi-codeword MIMO transmission transport format and/or HARQ information for multiple code words may be included.
For uplink resource assignments (on the Physical Uplink Shared Channel (PUSCH)) signaled on PDCCH in LTE, the L1/L2 control information does not contain a HARQ process number, since a synchronous HARQ protocol is employed for LTE uplink. The HARQ process to be used for an uplink transmission is given by the timing. Furthermore, it should be noted that the redundancy version (RV) information is jointly encoded with the transport format information, i.e., the RV info is embedded in the transport format (TF) field. The Transport Format (TF) respectively modulation and coding scheme (MCS) field has for example a size of 5 bits, which corresponds to 32 entries. 3 TF/MCS table entries are reserved for indicating redundancy versions (RVs) 1, 2 or 3. The remaining MCS table entries are used to signal the MCS level (TBS) implicitly indicating RV0. The size of the CRC field of the PDCCH is 16 bits.
For downlink assignments (PDSCH) signaled on PDCCH in LTE the Redundancy Version (RV) is signaled separately in a two-bit field. Furthermore the modulation order information is jointly encoded with the transport format information. Similar to the uplink case there is 5 bit MCS field signaled on PDCCH. 3 of the entries are reserved to signal an explicit modulation order, providing no Transport format (Transport block) info. For the remaining 29 entries modulation order and Transport block size info are signaled.
Channel Quality Reporting
The principle of link adaptation is fundamental to the design of a radio interface which is efficient for packet-switched data traffic. Unlike the early versions of UMTS (Universal Mobile Telecommunication System), which used fast closed-loop power control to support circuit-switched services with a roughly constant data rate, link adaptation in LTE adjusts the transmitted data rate (modulation scheme and channel coding rate) dynamically to match the prevailing radio channel capacity for each user.
For the downlink data transmissions in LTE, the eNodeB typically selects the modulation scheme and code rate (MCS) depending on a prediction of the downlink channel conditions. An important input to this selection process is the Channel State Information (CSI) feedback transmitted by the User Equipment (UE) in the uplink to the eNodeB.
Channel state information is used in a multi-user communication system, such as for example 3GPP LTE to determine the quality of channel resource(s) for one or more users. In general, in response to the CSI feedback the eNodeB can select between QPSK, 16-QAM and 64-QAM schemes and a wide range of code rates. This CSI information may be used to aid in a multi-user scheduling algorithm to assign channel resources to different users, or to adapt link parameters such as modulation scheme, coding rate or transmit power, so as to exploit the assigned channel resources to its fullest potential.
The CSI is reported for every component carrier, and, depending on the reporting mode and bandwidth, for different sets of subbands of the component carrier. In 3GPP LTE, the smallest unit for which channel quality is reported is called a subband, which consists of multiple frequency-adjacent resource blocks.
As described before, user equipments will usually not perform and report CSI measurements on configured but deactivated downlink component carriers but only radio resource management related measurements like RSRP (Reference Signal Received Power) and RSRQ (Reference Signal Received Quality).
Commonly, mobile communication systems define special control signaling that is used to convey the channel quality feedback. In 3GPP LTE, there exist three basic elements which may or may not be given as feedback for the channel quality. These channel quality elements are:
MCSI: Modulation and Coding Scheme Indicator, sometimes referred to as Channel Quality Indicator (CQI) in the LTE specification
PMI: Precoding Matrix Indicator
RI: Rank Indicator
The MCSI suggests a modulation and coding scheme that should be used for transmission, while the PMI points to a pre-coding matrix/vector that is to be employed for spatial multiplexing and multi-antenna transmission (MIMO) using a transmission matrix rank that is given by the RI. Details about the involved reporting and transmission mechanisms are given in the following specifications to which it is referred for further reading (all documents available at http://www.3gpp.org and incorporated herein by reference):
3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation”, version 10.0.0, particularly sections 6.3.3, 6.3.4,
3GPP TS 36.212, “Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and channel coding”, version 10.0.0, particularly sections 5.2.2, 5.2.4, 5.3.3,
3GPP TS 36.213, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures”, version 10.0.1, particularly sections 7.1.7, and 7.2.
In 3GPP LTE, not all of the above identified three channel quality elements are reported at any time. The elements being actually reported depend mainly on the configured reporting mode. It should be noted that 3GPP LTE also supports the transmission of two codewords (i.e., two codewords of user data (transport blocks) may be multiplexed to and transmitted in a single sub-frame), so that feedback may be given either for one or two codewords. The individual reporting modes for the aperiodic channel quality feedback are defined in 3GPP LTE.
The periodicity and frequency resolution to be used by a UE to report on the CSI are both controlled by the eNodeB. The Physical Uplink Control Channel (PUCCH) is used for periodic CSI reporting only (i.e., CSI reporting with a specific periodicity configured by RRC); the PUSCH is used for aperiodic reporting of the CSI, whereby the eNodeB specifically instructs (by a PDCCH) the UE to send an individual CSI report embedded into a resource which is scheduled for uplink data transmission.
In addition, in case of multiple transmit antennas at the eNodeB, CSI values(s) may be reported for a second codeword. For some downlink transmission modes, additional feedback signaling consisting of Precoding Matrix Indicators (PMI) and Rank Indications (RI) is also transmitted by the UE.
In order to acquire CSI information quickly, eNodeB can schedule aperiodic CSI by setting a CSI request bit in an uplink resource grant sent on the Physical Downlink Control Channel.
In 3GPP LTE, a simple mechanism is foreseen to trigger the so-called aperiodic channel quality feedback from the user equipment. An eNodeB in the radio access network sends a L1/L2 control signal to the user equipment to request the transmission of the so-called aperiodic CSI report (see 3GPP TS 36.212, section 5.3.3.1.1 and 3GPP TS 36.213, section 7.2.1 for details). Another possibility to trigger the provision of aperiodic channel quality feedback by the user equipments is linked to the random access procedure (see 3GPP TS 36.213, section 6.2).
Whenever a trigger for providing channel quality feedback is received by the user equipment, the user equipment subsequently transmits the channel quality feedback to the eNodeB. Commonly, the channel quality feedback (i.e., the CSI report) is multiplexed with uplink (user) data on the Physical Uplink Shared CHannel (PUSCH) resources that have been assigned to the user equipment by L1/L2 signaling by the scheduler (eNodeB). In case of carrier aggregation, the CSI report is multiplexed on those PUSCH resources that have been granted by the L1/L2 signal (i.e., the PDCCH) which triggered the channel quality feedback.
Sounding Reference Symbol (SRS)
The SRS are important for uplink channel sounding to support dynamic uplink resource allocation, as well as for reciprocity-aided beamforming in the downlink. Release 10 introduces the possibility of dynamically triggering individual SRS transmissions via the PDCCH; these dynamic aperiodic SRS transmissions are known as “type-1” SRSs, while the Release 8 periodic RRC-configured SRSs are known as “type-0” in Release 10.
An indicator in an uplink resource grant on the PDCCH can be used to trigger a single type 1 SRS transmission. This facilitates rapid channel sounding to respond to changes in traffic or channel conditions, without typing up SRS resources for a long period. In DCI format 0, one new bit can indicate activation of a type 1 SRS according to a set of parameters that is configured beforehand by RRC signaling. In DCI format 4, which is used for scheduling uplink SU-MIMO transmissions, two new bits allow one of three sets of RRC-configured type 1 SRS transmission parameters to be triggered.
The SRS transmissions are always in the last SC-FDMA symbol of the corresponding subframe where reporting is configured/scheduled. PUSCH data transmission is not permitted on the SC-FDMA signal designated for SRS, i.e., PUSCH transmission is punctured such that all symbols but the last are used for PUSCH.
Uplink Control Signaling and Multiplexing
When simultaneous uplink PUSCH data and control signaling are scheduled, the control signaling is normally multiplexed together with the data (in PUSCH) prior to the DFT spreading, in order to preserve the single-carrier low Cubic Metric (CM) property of the uplink transmission. The uplink control channel, PUCCH, is used by a UE to transmit any necessary control signaling only in subframes in which the UE has not been allocated any RBs for PUSCH transmission.
Further information on the multiplexing of the uplink control signaling can be found in Chapters 16.3.1.1, 16.3.3, 16.3.4, 16.3.5, 16.3.6, 16.3.7, 16.4 of LTE—The UMTS Long Term Evolution—From Theory to Practice, Edited by Stefanie Sesia, Issam Toufik, Matthew Baker, Second Edition, incorporated herein by reference
DRX (Discontinuous Reception)
In order to provide reasonable battery consumption of user equipment, 3GPP LTE (Release 8/9) as well as 3GPP LTE-A (Release 10) provides a concept of discontinuous reception (DRX). Technical Standard TS 36.321 Chapter 5.7 explains the DRX and is incorporated by reference herein.
The following parameters are available to define the DRX UE behavior; i.e., the periods at which the mobile node is active (i.e., in Active Time), and the periods where the mobile node is not active (i.e., in Non-Active Time, while in DRX mode).
On duration (timer): duration in downlink sub-frames that the user equipment, after waking up from DRX (Non-Active Time), receives and monitors the PDCCH. If the user equipment successfully decodes a PDCCH, the user equipment stays awake and starts the DRX Inactivity Timer; [1-200 subframes; 16 steps: 1-6, 10-60, 80, 100, 200]
DRX Inactivity Timer: duration in downlink sub-frames that the user equipment waits to successfully decode a PDCCH, from the last successful decoding of a PDCCH; when the UE fails to decode a PDCCH during this period, it re-enters DRX. The user equipment shall restart the DRX Inactivity Timer following a single successful decoding of a PDCCH for a first transmission only (i.e., not for retransmissions). [1-2560 subframes; 22 steps, 10 spares: 1-6, 8, 10-60, 80, 100-300, 500, 750, 1280, 1920, 2560]
DRX Retransmission timer: specifies the number of consecutive PDCCH subframes where a downlink retransmission is expected by the UE after the first available retransmission time. [1-33 subframes, 8 steps: 1, 2, 4, 6, 8, 16, 24, 33]
DRX short cycle: specifies the periodic repetition of the on duration followed by a possible period of inactivity for the short DRX cycle. This parameter is optional. [2-640 subframes; 16 steps: 2, 5, 8, 10, 16, 20, 32, 40, 64, 80, 128, 160, 256, 320, 512, 640]
DRX short cycle timer: specifies the number of consecutive subframes the UE follows the short DRX cycle after the DRX Inactivity Timer has expired. This parameter is optional.[1-16 subframes]
Long DRX Cycle Start offset: specifies the periodic repetition of the on duration followed by a possible period of inactivity for the DRX long cycle as well as an offset in subframes when on-duration starts (determined by formula defined in TS 36.321 section 5.7); [cycle length 10-2560 subframes; 16 steps: 10, 20, 30, 32, 40, 64, 80, 128, 160, 256, 320, 512, 640, 1024, 1280, 2048, 2560; offset is an integer between [0-subframe length of chosen cycle]]
The total duration that the UE is awake is called “Active time”. The Active Time includes the OnDuration time of the DRX cycle, the time UE is performing continuous reception while the DRX Inactivity Timer has not expired and the time UE is performing continuous reception while waiting for a downlink retransmission after one HARQ RTT. Similarly for the uplink, UE is awake at the subframes where Uplink retransmissions grants can be received, i.e., every 8 ms after initial uplink transmission until maximum number of retransmissions is reached. Based on the above the minimum active time is of length equal to on-duration, and the maximum is undefined (infinite). Furthermore also after having sent an SR on the PUCCH UE will be awake monitoring for a PDCCH allocating UL-SCH Conversely, the Non-Active Time is basically the duration of downlink subframes during which a UE can skip reception of downlink channels for battery saving purposes.
The operation of DRX gives the mobile terminal the opportunity to deactivate the radio circuits repeatedly (according to the currently active DRX cycle) in order to save power. Whether the UE indeed remains in Non-Active Time (i.e., is not active) during the DRX period may be decided by the UE; for example, the UE usually performs inter-frequency measurements which cannot be conducted during the On-Duration, and thus need to be performed some other time.
The parameterization of the DRX cycle involves a trade-off between battery saving and latency. On the one hand, a long DRX period is beneficial for lengthening the UE's battery life. For example, in the case of a web browsing service, it is usually a waste of resources for a UE continuously to receive downlink channels while the user is reading a downloaded web page. On the other hand, a shorter DRX period is better for faster response when data transfer is resumed—for example when a user requests another web page.
To meet these conflicting requirements, two DRX cycles—a short cycle and a long cycle—can be configured for each UE. The transition between the short DRX cycle, the long DRX cycle and continuous reception is controlled either by a timer or by explicit commands from the eNB. In some sense, the short DRX cycle can be considered as a confirmation period in case a late packet arrives, before the UE enters the long DRX cycle—if data arrives at the eNB while the UE is in the short DRX cycle, the data is scheduled for transmission at the next wake-up time and the UE then resumes continuous reception. On the other hand, if no data arrives at the eNB during the short DRX cycle, the UE enters the long DRX cycle, assuming that the packet activity is finished for the time being.
Available DRX values are controlled by the network and start from non-DRX up to x seconds. Value x may be as long as the paging DRX used in IDLE. Measurement requirement and reporting criteria can differ according to the length of the DRX interval, i.e., long DRX intervals may experience more relaxed requirements.
When DRX is configured, periodic CQI/SRS reports shall only be sent by the UE during the “active-time”. RRC can further restrict periodic CQI reports so that they are only sent during the on-duration.
In FIG. 8 a per-subframe example of the DRX cycle is shown. The UE checks for scheduling messages (indicated by its C-RNTI on the PDCCH) during the ‘On Duration’ period of either the long DRX cycle or the short DRX cycle depending on the currently active cycle. When a scheduling message is received during an ‘On Duration’, the UE starts an ‘Inactivity Timer’ and monitors the PDCCH in every subframe while the Inactivity Timer is running. During this period, the UE can be regarded as being in a continuous reception mode. Whenever a scheduling message is received while the Inactivity Timer is running, the UE restarts the Inactivity Timer, and when it expires the UE moves into a short DRX cycle and starts a ‘Short DRX cycle timer’. The short DRX cycle may also be initiated by means of a DRX MAC Control Element from the eNodeB, instructing the UE to enter DRX. When the short DRX cycle timer expires, the UE moves into a long DRX cycle. In addition to this DRX behavior, a ‘HARQ Round Trip Time (RTT) timer’ is defined with the aim of allowing the UE to sleep during the HARQ RTT. When decoding of a downlink transport block for one HARQ process fails, the UE can assume that the next retransmission of the transport block will occur after at least ‘HARQ RTT’ subframes. While the HARQ RTT timer is running, the UE does not need to monitor the PDCCH. At the expiry of the HARQ RTT timer, the UE resumes reception of the PDCCH as normal.
Above mentioned DRX related timers like DRX-Inactivity timer, HARQ RTT timer, DRX retransmission timer and Short DRX cycle timer are started and stopped by events such as reception of a PDCCH grant or MAC Control element (DRX MAC CE); hence the DRX status (active time or non-active time) of the UE can change from one subframe to another and is hence not always predictable by the mobile station or eNodeB.
There is only one DRX cycle per UE. All aggregated component carriers follow this DRX pattern.
Shortcomings of Current Periodic CSI/SRS Reporting During DRX
As mentioned before, the DRX status (i.e., Active Time/non-Active Time) of a UE can change from subframe to subframe. DRX-related timers (like DRX-Inactivity timer, HARQ RTT timer, DRX retransmission timer) are started and stopped by various events, such as reception of a PDCCH grant or of MAC control elements (DRX MAC CE), thus putting the UE into Active Time or non-Active Time. The behavior of the UE for Active Time and non-Active Time is clearly defined by the standard. Correspondingly, the UE shall transmit periodic CSI reports and SRS only during the Active time. However, the UE needs some time to process received signaling or information changing its DRX status, and also need some time to prepare the CSI report and SRS. The processing time strongly depends on the implementation of the UE. This however may lead to problems during operation of the UE, as will be explained in detail below.
Assuming the UE is currently in Active Time and the DRX Inactivity timer is running, if a UE receives in the last subframe before the DRX Inactivity timer expires (e.g., subframe N) a PDCCH indicating a new transmission (UL or DL), the UE will also be in Active Time in the next subframe, i.e., subframe N+1 and the DRX Inactivity timer is restarted.
Due to the processing time in the UE, the UE may only now at the beginning/middle of subframe N+1 that subframe N+1 is still Active Time. Assuming that the periodic CSI report is configured to be transmitted in subframe N+1, the UE may not have time to prepare the CSI report for transmission, since it initially assumed to enter DRX, i.e., be in non-Active Time during subframe N+1, and thus to not be necessary to transmit the CSI report. Consequently, the UE might not be able to transmit the periodic CSI report in subframe N+1, contrary to the specification mandating the UE to transmit periodic CSI on PUCCH during Active Time in the configured subframes.
In summary, the UE behavior with respect to CSI/SRS transmission cannot immediately follow the DRX status of the UE, since the UE needs some time to become aware of the signaling and to prepare the necessary uplink transmission accordingly. The time after the Active Time has been suddenly started/prolonged or ended due to reception of respective signaling from the network is generally referred to as “transient phase” or “uncertain period”. In order to account for the processing delay in the UE, an exception on the periodic CSI transmission on PUCCH and periodic SRS transmission has been introduced for LTE Rel-8/9/10 in TS 36.321, as follows.                A UE may optionally choose to not send CQI/PMI/RI/PTI reports on PUCCH and/or type-0-triggered SRS transmissions for up to 4 subframes following a PDCCH indicating a new transmission (UL or DL) received in subframe n−i, where n is the last subframe of Active Time and i is an integer value from 0 to 3. After Active Time is stopped due to the reception of a PDCCH or a MAC control element a UE may optionally choose to continue sending CQI/PMI/RI/PTI reports on PUCCH and/or SRS transmissions for up to 4 subframes. The choice not to send CQI/PMI/RI/PTI reports on PUCCH and/Or type-0-triggerred SRS transmissions is not applicable for subframes where onDurationTimer is running and is not applicable for subframes n−i to n.        
Despite the above exception, the eNB in general expects uplink transmissions from the UE according to the specification. Thus, with respect to CSI/SRS reporting, when the UE is in Active Time, the UE is expected to transmit periodic CSI reports on PUCCH and SRS, depending on the periodicity of CSI/SRS. Correspondingly, the eNB does not expect any periodic CSI/SRS transmission from UE in subframes where the UE is in non-Active Time.
However, due to the UE behavior introduced to cover the “transient phases”, the UE behavior for these “transient phases” is not predictable for the eNB. Therefore, the network must be able to correctly decode the PUCCH channel or the PUSCH channel for cases, when it does not know if periodic CSI or SRS reports have been sent or not. In other words, double decoding is necessary at the UE to cover both transmission cases, i.e., with or without CSI/SRS. For instance:
If CSI happens to coincide with a DL HARQ PUCCH transmission in the transient phase, then, the network needs to perform double decoding to handle both the case, when CSI has been sent and the case when CSI has not been sent.
If SRS happens to coincide with a PUSCH transmission that is outside the configured bandwidth of SRS in the transient phase, then the network needs to perform double decoding to handle both the case when SRS has been sent and the case when SRS has not been sent.
There are many more combinations of control information for which eNB needs to perform double decoding for two different data transmissions formats in order to be able to detect the control information correctly. Some of these combinations are given in the table below, which is taken from R2-124687; it should be noted that the list is not complete, but shall give an overview.
Case (possibleDoublecollisions duringIf CSI/SRS isIf CSI/SRS is notdecodingtransient phase)transmittedtransmittedneeded?CSI + DataData (RMed) + CSIDataYesCSI + ANCSI + AN (jointlyANYescoded)CSI + SRSR (CSI dropped)SRNoCSI + Data + SRData (RMed) + CSIDataYesCSI + Data + AN[CSI & Data Muxed]Data (RMed) + ANYes(RMed) + ANCSI + AN + SRAN + SRAN + SRNoCSI + Data + AN + SR[CSI & Data Muxed]Data (RMed) + ANYes(RMed) + ANSRS + DataData (RMed) + SRSDataYesSRS + AN[AN (shorten format) +AN (shorten format) orNoSRS] or AN (normalAN (normal format)format)SRS + SR[SR (shorten format) +SR (shorten format) orNoSRS] or SR (normal format)SR (normal format)SRS + Data + SRData (RMed) + SRSDataYesSRS + Data + ANData (RMed overData (RMed over AN) +YesAN/SRS) + AN + SRSANSRS + AN + SR[AN + SR] (shorten[AN + SR] (shortenNoformat) + SRS or [AN +format) or [AN + SR]SR] (normal format)(normal format)SRS + Data + AN + SRData (RMed overData (RMed over AN) +YesAN/SRS) + AN + SRSANCSI + SRS + DataData (RMed overData (RMed over CSI) +YesCSI/SRS) + CSI + SRSCSICSI + SRS + ANAN (shorten format) +AN (shorten format) orNoSRS or AN (normalAN (normal format)format)CSI + SRS + SRSR (shorten format) +SR (shorten format)NoSRSCSI + SRS + Data + SRData (RMed overData (RMed over CSI) +YesCSI/SRS) + CSI + SRSCSICSI + SRS + Data + AN[CSI & Data Muxed]Data (RMed over AN) +Yes(RMed over AN/SRS) +ANAN + SRSCSI + SRS + AN + SRAN + SR (shorten format) + SRSAN + SR (normal format)YesCSI + SRS + Data + AN + SR[CSI & Data Muxed]Data (RMed over AN) +Yes(RMed over AN/SRS) +ANAN + SRS
As can be seen, the double decoding caused by the transient phases might happen quite often, and causes unnecessary complexity and computational cost within the network. The decoding in the eNB relies on the uplink transmissions having a certain transmission format, as for example Format 2, 2a and 2b always including a CSI. When the transmission format changes due to the sudden transmission or non-transmission of the CSI, the decoding in the eNB may fail due to the wrong transmission format, which in turn leads to degradation of the throughput.
This applies in a similar manner for the transmission of the SRS. Provided the assigned resource blocks for PUSCH are not overlapping with the cell-specific SRS frequency region, in case the UE doesn't transmit SRS in this subframe, the UE uses the last SC-FDMA symbol in the subframe for PUSCH. In case the UE transmit SRS in this subframe, the UE does not use the last SC-FDMA symbol for PUSCH. Therefore, depending on whether UE is transmitting SRS (which is dependent on the DRX status of the subframe), the number of SC-FDMA symbols for PUSCH changes, which in turn means that eNB would have to check two different PUSCH symbol usages in those subframes. However, this uncertainty can be easily avoided by the eNB by assigning only PUSCH resources to the UE which lie within the cell-specific SRS region, which is majority of the assignment; in this case the UE will never map PUSCH on the last SC-FDMA symbol in a subframe where periodic SRS has been configured. Nevertheless, the problem remains for the case where the assigned resource blocks for the PUSCH do not lie within the cell-specific SRS region.